(1) Field of the Invention
The present invention relates generally to an improved tap useful in rethreading blind tapped holes having damaged threads, and in particular to a rethreading tap that can be inserted into a hole with damaged threads so that the tap cutting teeth are positioned beneath the damaged area and then expanded for repair of the damaged threads.
(2) Description of the Prior Art
Blind threaded holes are commonly formed in metal, plastic or other materials by drilling a hole, and then inserting the distal end of a tap having a diameter slightly larger than the diameter of the drilled hole. The tap is slowly rotated with a T-shaped handle that is attached to the upper end of the tap with a collet or chuck, taking care to keep the tap aligned with the hole. Rotation of the tap clockwise cuts threads in the face of the hole. Periodically, the tap is rotated counterclockwise to remove debris.
Holes that do not extend entirely through the material are referred to as blind holes. In use, a threaded bolt, threaded rod, or the like, with threads matching the hole threads is screwed into the hole. Over time, the blind hole threads may be damaged, often by a user attempting to screw in a misaligned bolt, by impact on the bolt head, or other reasons. Often, the thread damage is restricted to the surface end of the hole, while the threads further in the hole remain undamaged.
Repair of the hole, if possible, often requires redrilling and tapping of a larger diameter hole, which in turn requires the use of a larger diameter bolt. Alternatively, the blind hole may be drilled out so that a threaded insert, can be fitted into the hole.
Neither of these solutions is desirable. Use of a larger diameter bolt may not be possible due to the requirement of other components. Fitting of an insert is time consuming, expensive, and does not have the strength of a drilled hole. Therefore, there is a continuing need for a method and device that will enable rethreading of a damaged threaded, blind hole, particularly where the thread damage is near the surface of the hole.